In recent years, a demand for 4,4'-DDS has increased in the chemical industry, e.g. fields of fibers, resins or the like. Furthermore, there has arisen a need for an extremely high-purity 4,4'-DDS to improve the quality of products in each field.
A process for preparation of 4,4'-DDS is known in which a phenol and a sulfonating agent are subjected to dehydration reaction in the presence of a solvent such as dichlorobenzene which is capable of dissolving the foregoing materials and the produced 4,4'-DDS. However, when the dehydration reaction is conducted in a solvent as in said process, the desired 4,4'-DDS as dissolved in the reaction product has an isomerization equilibrium with an isomer that is, 2,4'-dihydroxydiphenylsulfone (hereinafter referred to as "2,4'-DDS") is produced as a by-product so that the obtained crude product contains 20 to 30% by weight of 2,4'-DDS as the impurity which decreases the purity and the yield of 4,4'-DDS. Further, it has been recently recognized that trihydroxytriphenyldisulfone (hereinafter referred to as "tri-compound") is produced as a by-product along with 2,4'-DDS.
In view of the above problems, the present inventors proposed a process for preparing a high-purity 4,4'-DDS in a high yield in which a phenol and sulfuric acid undergo dehydration reaction in the presence of a solvent, and the 2,4'-DDS produced as a by-product is isomerized into 4,4'-DDS while gradually removing the solvent from the reaction mixture (Japanese Examined Patent Publication No.55-8972). The proposed process is intended to increase the purity and the yield of 4,4'-DDS in the following manner. In the process, only the 4,4'-DDS is precipitated from the reaction system by gradually removing the solvent utilizing the difference between 4,4'-DDS and 2,4'-DDS in the solubility in the solvent, whereby the isomerization equilibrium is shifted in the solution to induce the isomerization of 2,4'-DDS into 4,4'-DDS. However, on removal of the solvent, the process causes the solidification of the reaction product solution into a non-fluidic viscous solid, and thus necessitates a special type of stirrer of high mechanical strength, resulting in difficulty in provision of a large-size manufacturing apparatus for mass production.
Recently proposed is a process in which while a phenol and sulfuric acid are subjected to dehydration reaction with heating in the presence of an aliphatic hydrocarbon suspending agent and an azeotropic agent, the 2,4'-DDS produced in the reaction is isomerized into 4,4'-DDS (Japanese Unexamined Patent Publication No.64-9970). However, the proposed process necessitates isomerization at a high temperature under fully controlled temperature conditions over a prolonged period of time. Furthermore, since the 2,4'-DDS is isomerized into 4,4'-DDS as suspended in a suspending agent, the process requires a large-size manufacturing apparatus and is unavoidably uneconomical from a thermal standpoint.